Buy a $20 or so router. Connect her machine through it. Any NetGear, Linksys, or other knockoff has the ability to limit specific machines to, or from certain sites during specific hours.
This is from my WRT54GL with the free third party Thibor15c firmware. It has a few neat addons that the default software doesn't.. but this feature is standard, hell my $20 AT&T branded router had it in 2003:
